Psychological and methodological issues in substance abuse treatment/research are under investigation as part of the clinical trials evaluating cocaine and opioid abuse treatments. To evaluate baseline drug use as a predictor of treatment outcome, cocaine use (qualitative and quantitative urinalysis and self-report) during a 5-week baseline was compared in methadone maintenance patients who had less than five versus more than 5 weeks of abstinence during a 12-week experimental voucher-based cocaine abstinence reinforcement treatment. Cocaine use was evaluated at the first and last clinic visit and first and last week of baseline and as a mean across the 5-week baseline; treatment response was calculated as a mean across 12-weeks of treatment. Those who had successful outcomes (Abstainers) used significantly less cocaine in the 5-week baseline than those with less successful outcomes (Nonabstainers). Differences in cocaine use were not evident in the first baseline visit or week, but Abstainers used significantly less cocaine in the last visit and week of baseline compared to Nonabstainers. Qualitative urinalysis was least sensitive to baseline differences. Thus, cocaine use during baseline provided critical predictors of response to the experimental treatment. Information on predictors of treatment outcome will improve patient/treatment matching and be useful for allocating treatment resources. As part of ongoing research efforts to improve methods of monitoring drug use in treatment patients, the presence of nonmetabolized cocaine in urine specimens was evaluated as a possible marker for recent, heavy cocaine use. Urine specimens collected during a cocaine treatment clinical trial were tested. Cocaine was measured by gas chromatography-mass spectrometry, and benzoylecgonine (cocaine metabolite) equivalents were determined by fluorescence polarization immunoassay. More than one-third of the specimens were positive for cocaine, and nearly two-thirds were positive for cocaine metabolite. Cocaine was present less frequently and at lower concentrations than benzoylecgonine, but more frequently than expected based on an average half-life of approximately 1 hour, suggestive that cocaine may exhibit a longer terminal half-life and/or that accumulation of cocaine can occur in chronic, heavy users. More work is needed to characterize the pharmacokinetics of cocaine in heavy users. This information may be useful for improving our understanding of the effects of cocaine in heavy users and for detecting decreases in cocaine use.